The present invention relates to vibration welding machinery, and more particularly to an apparatus and method for controlling the motion of a linear vibration welding device.
Vibration welding is used to join two workpieces made of thermoplastic. Items such as automobile bumpers, interior decorations, grilles, and lights are commonly formed using vibration welding techniques. An advantage of the vibration welding process is the reduced joining time when compared to adhesive bonding and heated tool welding.
Vibration welding works by frictionally working two plastic pieces under pressure, thereby heating and melting their contact surfaces. Once the whole surface is melted, reaching the so-called steady-state melt flow phase, the friction generating process is stopped and the parts form a bonded high-strength structure upon cooling.
Friction is generated by rubbing the two pieces together in an oscillatory fashion under pressure. There are two main types of vibration welding. The first, linear, involves one-dimensional oscillation of a workpiece. The second, orbital, involves biaxial oscillation of a workpiece. The range of oscillation frequencies used is typically between 80 and 300 Hz. In contrast, ultrasonic welding operates at frequencies of about 25 KHz. The amplitude of the oscillations for linear vibration welding is typically between 50 and 100 thousandths of an inch. The clamping force between the two parts is typically between 1000 and 5000 pounds force.
A linear vibration welding device most generally comprises a flexure member and a means for vibrating the flexure member. The prior art devices, such as U.S. Pat. No. 3,920,504, to Shoh et al., utilize one electromagnet at each end of the flexure array to generate a magnetic field to cause the flexure array to vibrate. These electromagnets are driven by a three-phase alternating current (AC) drive source, such as a variable frequency drive (VFD). This prior art AC drive system possesses several undesirable characteristics.
The use of three-phase AC power requires a large power input for a given amount of work output. Three-phase AC power possesses three poles separated by 120 degrees of phase between each pole. This makes AC well suited to rotary motion but not linear motion, which requires a 180 degree linear oscillatory motion. To make the AC system function, one of the two electromagnets receives both power coils, while the other magnet receives a single coil. A Scott T-connection is used, as shown in FIG. 10 of Shoh, to approximate a 180-degree phase alternation of the current. However, the approximated two-phase system does not eliminate all three-phase properties. Therefore, there is a series of counterproductive forces introduced to the system.
The counterproductive forces are those forces that urge the flexure array in a direction opposite that of its intended movement. Such forces work against the drive force, resulting in a net reduction in the drive force. A significantly larger drive is therefore required to achieve the necessary net drive force to weld a workpiece. The large drive consumes a correspondingly larger amount of power. Additionally, the frame for such device must be larger and heavier to handle the competing forces without premature failure.
The startup time for the prior art AC drive system is also disadvantageously lengthy. Startup time is the time it takes the machine to reach a constant maximum amplitude at the resonance frequency for the system. The startup time directly affects the welding process. Vibration speeds of about 35 inches per second and higher cause melting for most plastics. Speeds below about 20 inches per second will only cause the material to heat, not melt. The vibration speeds between these two values cause considerable amounts of particulates to be generated. This may cause poor welds, environmental concerns, machine interference and mess.
The use of three-phase AC power also disadvantageously requires the use of an autotuning system. The spring constants for flexure arrays used in vibration welders are very high, such as several hundred thousand pounds force per inch. Consequently, the flexure array will only move at or around its resonance frequency. This resonance frequency varies with the weight of the tool attached to the array. Therefore, the welding device must be xe2x80x9ctunedxe2x80x9d prior to use with a given tool.
The tuning step for conventional vibration welders relies on approximation based upon the user""s best guess. The operator simply varies the frequency input to the drive motor and listens to the audible hum. When the hum reaches its loudest point, the operator assumes that the amplitude has peaked.
An autotuning procedure became feasible with the advent of cost effective controls. Autotuning comprises the provision of an amplitude sensor and automated frequency adjustment controls to the welding apparatus. The frequency is first xe2x80x9cturned onxe2x80x9d at a predetermined starting level with a low power input. Then the frequency is stepped in increments of approximately 0.1 Hz while the sensor measures the amplitude. At the point where the amplitude begins to drop off, the stepping is discontinued. From the plot of amplitude versus frequency (at a fixed power level), the operating frequency is chosen where the peak displacement occurred.
A so-called soft start is used when autotuning. The power input is initially started low to ensure that the flexure member does not overextend and damage the drive magnets. Once the resonance frequency is determined, the power input is then increased to achieve a desired amplitude. This autotuning procedure adds time to the welding process, which reduces productivity.
An alternative method of autotuning is to introduce a known frequency to the system and monitor how it responds. The response is measured. Then a resonance frequency can be determined based upon the measured response. This method of autotuning exhibits the same deficiencies as the above-described stepping method.
The drive frequency of the prior art apparatus cannot be easily varied during a welding procedure. The viscosity of the interface between two work pieces being joined by vibration welding varies with the temperature and matter phase of the interface between the pieces. The viscosity may either increase or decrease, depending on the properties of the materials being joined, during a given weld procedure. The amplitude will increase given a decrease viscosity and constant power and frequency inputs. The opposite is true for increasing viscosity. Therefore, the prior art AC devices must vary one of the power or frequency inputs to the system to ensure that the amplitude is kept within a range to prevent damage to the machine and to ensure a good weld.
The prior art mechanisms do not have the ability to vary frequency during the weld process, so the power must be adjusted. The power rating of the drive mechanisms must be sufficiently oversized to allow for increased power needs of the system. Larger drive motors increase the cost of the overall apparatus.
The amplitude adjustment of the prior art devices is reactionary. The controller uses position information to compare the allowable amplitude range to a measured amplitude value. The controller is then able to determine whether the amplitude value is over or under the pre-set amplitude. The controller varies the power input to the drive motors to correct for the over or under amplitude condition. Then the amplitude is again compared to determine if the correction brought the amplitude back into a proper range.
This prior art reactionary method of adjusting the amplitude involves a considerable lag time between initial apprehension of the out of bounds condition until the condition is corrected. Several periods of flexure travel may occur before the problem is corrected. This lag in response time can have adverse effects on both the workpiece and on the apparatus itself. Some thermoplastic materials used in vibration welding processes can change viscosities very rapidly during a joining process. Because of this quick change and lag in apparatus adjustment, damage to the workpiece and the drive magnets can occur due to an over-amplitude condition.
Finally, the prior art three-phase AC drive vibration welders do not provide for the ability to weld by energy. Welding by energy, as is often used in ultra-sonic welding, involves inputting a known amount of energy into the workpiece to create a weld. Welding by energy requires knowing how much energy is inputted in to the system and what percentage of that energy actually goes into the given workpiece. True weld by energy cannot be used with a three-phase AC system because one cannot easily measure the deductions necessary to account for the counterproductive forces.
In summary, conventional vibration welders have several significant disadvantages. Their AC power systems require large and costly drive motors, the frame must be correspondingly large and the overall system is slow to come up to speed. The AC drive system requires an autotuning function with a soft start. The method of adjusting the amplitude is reactionary and there is no method for welding by power. Additionally, the prior art apparatuses tend to be complex, costly and inefficient. Therefore, there is a need to provide a method and apparatus for vibration welding that addresses these disadvantages in whole or in part.
Disclosed are a method for controlling a linear vibration welding apparatus and an apparatus for same. The method, in accordance with the invention, may comprise the steps of: fastening a first workpiece portion in a fixed position; fastening a second workpiece portion to a reciprocating member; energizing a first single winding magnet with direct current power to create a magnetic field; sensing a location of the reciprocating member with respect to a zero point; and energizing a second magnet when the reciprocating member has crossed the zero point when moving towards the first magnet. The linear vibration welding apparatus in accordance with the invention may comprise: a frame; a flexure array; a first magnet assembly; a second magnet assembly; a digital controller; and direct current amplifiers for powering the magnet assemblies.
The present invention addresses the disadvantages present in conventional linear vibration welders. The present invention possesses increased efficiency by driving the electromagnet assemblies with direct current. The use of direct current eliminates the counterproductive forces present in three phase AC drive systems. The increased efficiency allows the apparatus to perform with approximately twice the welding power relative to a comparably sized conventional linear vibration welder. The DC drive system, in conjunction with digital controls, allows for dynamic modulation and predictive adjustment of the amplitude of the flexure array during a welding process. This eliminates the need for autotuning of the apparatus and minimizes the risk of overdrive related damage. The digital controls also allow for welding by power to be implemented.